All quests in which the reward is units are not counted. When you try to complete them, the units do not appear. In the quest, the mark on the map where you need to do the quest disappears. Further quest completion becomes impossible.
Update:
It turns out that the quest doesn't work when the quest requires you to explore the ruins. Whenever you explore the ruins with a quest for the first time, nothing happens, and the second time there is a normal exploration. It is not possible to complete quests with ruins.
